Sheathed-element glow plug

ABSTRACT

A sheathed-element glow plug to be mounted in a combustion chamber is proposed, a rod-shaped heating element being situated in a concentric bore hole of the housing. The heating element has a first current-carrying layer ( 11 ), a second current-carrying layer ( 12 ), and an insulating layer ( 15 ), the insulating layer ( 15 ) separating the first current-carrying layer ( 11 ) and the second current-carrying layer ( 12 ). The first current-carrying layer ( 11 ) and the second current-carrying layer ( 12 ) being connected at the end of the heating element on the combustion chamber side by a conducting-layer crosspiece ( 13 ). The first current-carrying layer ( 11 ) and the second current-carrying layer ( 12 ) are different lengths, the cross section of the first current-carrying layer ( 11 ) in a first section ( 21 ) at the end of the heating element away from the combustion chamber being greater than the cross section of its remaining length, and the second current-carrying layer ( 12 ) not extending into the first section ( 21 ).

BACKGROUND INFORMATION

[0001] The present invention starts out from a sheathed-element glowplug to be mounted in a combustion chamber, according to the speciesdefined in the independent claim. Sheathed-element glow plugs, having ametallic housing, to be mounted in a combustion chamber are alreadyknown. A rod-shaped heating element is situated in a concentric borehole of the known sheathed-element glow plug, the heating element havinga first current-carrying layer and a second current-carrying layer, thecross sections of the first and the second current-carrying layer beingconnected on the end of the heating element on the combustion chamberside via a conducting-layer crosspiece. In this context, the first andthe second current-carrying layers are separated by an insulating layer.Furthermore, sheathed-element glow plugs are known whosecurrent-carrying layers vary in length.

SUMMARY OF THE INVENTION

[0002] In contrast, the sheathed-element glow plug of the presentinvention having the features of the independent claim has the advantagethat there is no danger of a short circuit at the end of the heatingelement away from the combustion chamber. A farther advantage is thatthe contact surface between the first current-carrying layer and thecontact element situated at the end of the heating element away from thecombustion chamber is enlarged. The contact resistance is consequentlyreduced, thereby resulting in the contact point heating up less.Therefore, the danger of the contact material between the heatingelement and the contact element being thermally destroyed is decreased.In addition, it is advantageous that the rod-shaped heating element doesnot need to be adjusted to remove an insulating layer situated on therod-shaped heating element in the region in which the current supply isto be contacted.

[0003] The measures specified in the dependent claims render possible anadvantageous further refinement and improvement of the sheathed-elementglow plug indicated in the main claim. It is particularly advantageousto also design the insulating layer to be asymmetrical, so that thedanger of a short circuit due to damage or a porousness of an insulatinglayer deposited on the heating element is also decreased in thisinstance. In this context, it is advantageous to expand the region inwhich the insulating layer is asymmetrically formed in the direction ofthe combustion chamber over the collar of the heating element, since asummation of form-dependent and material-dependent stress concentrationis prevented in this manner. In addition, it is advantageous to designthe heating element such that an advantageously half-shell-shapedinsulating layer made of an electrically insulating, ceramic material isdeposited in the region in which the first current-carrying layerextends into the housing, the insulating layer between first and secondcurrent-carrying layer being made of the same material. As a result, themanufacturing process is simplified and, thus, more cost-effective. Inorder to be able to dispense with additional insulation, it isadvantageous to design the insulating layer such that it extends beyondthe end of the housing on the combustion chamber side. Furthermore, itis advantageous to provide a stepped lug at the end of the heatingelement away from the combustion chamber, so that an adapter sleevesituated on the end of the heating element away from the combustionchamber and the contact element are able to be easily positioned.

[0004] Further advantageous developments and improvements of thesheathed-element glow plug are to be inferred from the exemplaryembodiments shown below.

BRIEF DESCRIPTION OF THE DRAWING

[0005] Exemplary embodiments of the present invention are represented inthe drawings and are explained in detail in the following description.

[0006]FIG. 1 schematically shows a longitudinal section of asheathed-element glow plug according to the present invention,

[0007]FIGS. 2 through 4 schematically show a longitudinal section ofdifferent exemplary embodiments for a heating element of asheathed-element glow plug according to the present invention, and

[0008]FIG. 5 schematically shows a longitudinal section of the end of asheathed-element glow plug of the present invention on the combustionchamber side.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0009] The longitudinal section of a sheathed-element glow plug ofpresent invention is schematically shown in FIG. 1. The sheathed-elementglow plug has a housing 3, which is preferably made of a metallicmaterial, a heating element being situated in the housing's concentric,continuous bore hole, on the end on the combustion chamber side. Theheating element includes a first current-carrying layer 11, a secondcurrent-carrying layer 12, and an intermediary insulating layer 15.First current-carrying layer 11 and second current-carrying layer 12 areconnected by a conducting-layer crosspiece 13 at the end of the heatingelement on the combustion chamber side. The described configuration offirst current-carrying layer 11, second current-carrying layer 12, andconducting-layer crosspiece 13 results in a U-shaped configuration ofthe current-carrying layers. First current-carrying layer 11 iselectrically contacted on the end of the heating element away from thecombustion chamber to a contact element 31, which is preferably designedas a graphite pellet or another flexible and conductive element (e.g. ametal spring). Contact element 31 and the end of terminal stud 35 on thecombustion chamber side, the terminal stud being situated on the end ofcontact element 31 away from the combustion chamber, are situated in afirst adapter sleeve 33, first adapter sleeve 33 being the shape of ahollow cylinder and being made of an electrically insulating material.Terminal stud 35 extends to the end of the sheathed-element glow plugaway from the combustion chamber and runs in the inner, concentric borehole of housing 3. In this context, additional elements (second adaptersleeve 37 and metal ring 39), which are the shape of a hollow cylinderand through which terminal stud 35 runs, are situated in this bore hole.A contact plug 40 representing the connection to the glow plug switchingcircuit is placed on terminal stud 35 on the end away from thecombustion chamber. A sealing ring 41, which seals the inside of thehousing of the sheathed-element glow plug from the external space, issituated between housing 3 and contact plug 40. This sealing ring 41 isalso in the shape of a hollow cylinder.

[0010] The second current-carrying layer is electrically contacted via aregion in which electrically insulating layer 16, which surrounds theend of the heating element away from the combustion chamber, is removed,and also via sealant 5 to housing 3. Sealant 5 is situated around theend of the heating element away from the combustion chamber in the shapeof a ring and seals the inside of the housing in the direction of thecombustion chamber. In a preferred exemplary embodiment, a contact layer17 may also be deposited in the region in which the secondcurrent-carrying layer is to be contacted by sealant 5. Furthermore, acontact layer 17 on the end of the heating element away from thecombustion chamber may also produce the contact between firstcurrent-carrying layer 11 and contact element 31.

[0011] The construction of the heating element is to be described inmore detail on the basis of FIG. 2. FIG. 2 schematically shows thelongitudinal section of a heating element of a sheathed-element glowplug according to the present invention. Identical reference numeralsused in this and in the following figures in reference to FIG. 1designate identical elements. Therefore, this will not be discussedagain in detail. FIG. 2 shows that, on the end of the beating elementaway from the combustion chamber, in a first section 21, firstcurrent-carrying layer 11 has a cross section that is enlarged withrespect to the cross section of the remaining length of firstcurrent-carrying layer 11. As such, the longitudinal section of firstcurrent-carrying layer 11 has an asymmetrical L-shaped design. Theregions of first section 21 on the end of the heating element away fromthe combustion chamber that are not filled by first current-carryinglayer 11 are filled by insulating layer 15. Second current-carryinglayer 12 does not protrude into this first segment 21 of the heatingelement.

[0012] In a preferred exemplary embodiment, first current-carrying layer11 is so significantly enlarged in first section 21 that the crosssection of first current-carrying layer 11 in this section correspondsto the cross section of the heating element. The design of thisexemplary embodiment may also be seen in FIG. 1.

[0013] Widening the cross section of first current-carrying layer 11ensures an enlarged contact area between first current-carrying layer 11and contact element 31, which is situated at the end of the heatingelement away from the combustion chamber. This increase in the contactarea results in a decrease in the contact resistance and, thus, in aless significant heating of this region.

[0014] In the direction of the combustion chamber, a second section 22,in which the cross section of insulating layer 15 is asymmetricallyenlarged with respect to the cross section of its remaining length,i.e., the cross section in the direction of the combustion chamber,borders first section 21 of the heating element. Second current-carryinglayer 12 also does not extend into this second section 22. In thiscontext, the end of second section 22 on the side of the combustionchamber may be selected such that it is a component of theheating-element collar (see FIG. 1) or of the heating-element shaft (seeFIG. 2) or is situated exactly at junction 19 between theheating-element collar and the heating-element shaft. In this context,the region of the heating element having the greatest cross section atthe end away from the combustion chamber is referred to as theheating-element collar. The region of the heating element bordering theheating-element shaft in the direction of the combustion chamber and notbelonging to the heating-element collar is referred to as theheating-element shaft. Preferably, the end of second section 22 is notsituated such that it is precisely at junction 19 between theheating-element shaft and heating-element collar, since an additionalstress concentration due to the material junction at the particularlystressed point of the junction between the heating-element shaft and theheating-element collar is prevented in this manner. The proposedformation of insulating layer 15 effectively prevents a short circuitbetween first current-carrying layer 11 and second current-carryinglayer 12 at the end of the heating element away from the combustionchamber.

[0015] As shown in FIG. 2, an insulating layer 16, which is preferablydesigned as a glass coating, is situated at the end of the heatingelement away from the combustion chamber, in the region of the shell. Inregion 17, in which second current-carrying layer 12 is in electricalcontact with sealant 5, this insulating layer 16 is either interruptedor a contact layer 17, which improves the contact between secondcurrent-carrying layer 12 and sealant 5, is formed. This contact layer17 may preferably be designed as a metallic layer.

[0016]FIG. 3 schematically shows a longitudinal cross section of afurther exemplary embodiment of a heating element of a sheathed-elementglow plug of the present invention. This heating element does not havean insulating layer 16, which completely surrounds the end of theheating element on the combustion chamber side, but only an insulatinglayer 18 in the region in which the first current-carrying layer wouldbe in contact with housing 3 without insulating layer 18. Insulatinglayer 18 is preferably shaped like a half-shell. Section 23, in whichinsulating layer 18 is deposited, is referred to as third section 23 inthe following. In this context, it is advantageous when insulating layer18 extends from the end of the heating element away from the combustionchamber over the edge of the housing. Thus, a short circuit betweenfirst current-carrying layer 11 and housing 3 is effectively preventedas a result of the thickness of insulating layer 18 being up to several100 μm.

[0017] In a particularly preferred exemplary embodiment, insulatinglayer 18 is made from the same material as insulating layer 15.Consequently, a manufacturing process including the manufacture of alaminate of electrically insulating and electrically conductive ceramiclayers is possible in a particularly cost-effective manner, since alllayers are able to produced using the same systems and devices. Aprocess step involving depositing a chemically different layer isconsequently eliminated.

[0018]FIG. 4 schematically shows a further exemplary embodiment of aheating element of a sheathed-element glow plug according to the presentinvention. The heating element has on the end away from the combustionchamber a stepped lug 11′, which connects to first current-carryinglayer 11 and is made of the same material as first current-carryinglayer 11. This stepped lug is used to precisely place contacting element31 and first adapter sleeve 33, as also shown in FIG. 1.

[0019]FIG. 5 schematically shows a longitudinal cross section of afurther exemplary embodiment of a sheathed-clement glow plug accordingto the present invention. In this context, the figure is limited to theend of the sheathed-element glow plug on the combustion chamber side.This drawing is to be used to show again that insulating layer 18 orthird section 23 extends from the end of the heating element away fromthe combustion chamber and adjacent to the housing beyond the edge ofhousing 3 on the side of the combustion chamber. As already described inlight of FIG. 1, a first adapter sleeve 33 and a contact element 31connect to the end of the heating element away from the combustionchamber. In a preferred exemplary embodiment, the end face of lug 11′ ofsecond insulating layer 11 facing away from the combustion chamber maybe provided with a contact layer 17, which improves the contact betweenfirst insulating layer 11 and contact element 31.

[0020] In all of the exemplary embodiments, first current-carrying layer11, second current-carrying layer 12, and conducting-layer crosspiece 13are made of electrically conductive ceramic material. Insulating layer15 is made of electrically insulating material. The ceramic,electrically conductive and electrically insulating materials arepreferably ceramic composite structures including at least two of thecompounds Al₂O₃, MoSi₂, Si₃N₄, and Y₂O₃. These composite structures areable to be obtained using a one-step or multi-step sintering process.The specific resistivity of the layers may preferably be determined bythe MoSi₂ content and/or the particle size of the MoSi₂. The MoSi₂content of first and second current-carrying layers 11, 12 and ofconducting-layer crosspiece 13 is preferably greater than the MoSi₂content of insulating layer 15.

[0021] In a further exemplary embodiment, first and secondcurrent-carrying layers 11, 12, conducting-layer crosspiece 13, andinsulating layer 15 are made of a composite precursor ceramic havingdifferent proportions of fillers. In this context, the matrix of thismaterial is made of polysiloxanes, polysilsequioxanes, polysilanes, orpolysilazanes that may be doped with boron or aluminum and are producedby pyrolysis. At least one of the compounds Al₂O₃, MoSi₂, and SiC formsthe filler for the individual layers. Analogously to the abovementionedcomposite structure, the MoSi₂ content and/or the particle size of theMoSi₂ may preferably determine the specific resistivity of the layers.Preferably, the MoSi₂ content of first and second current-carryinglayers 11, 12 and of conducting-layer crosspiece 13 is greater than theMoSi₂ content of insulating layer 15.

[0022] The compositions of insulating layer 15, first and secondcurrent-carrying layers 11, 12, and conducting-layer crosspiece 13 areselected in the abovementioned exemplary embodiments such that theirthermal expansion coefficients and the shrinkage of the individual leadlayers, conducting-layer crosspiece layers, and insulating layersoccurring during the sintering or pyrolysis process are equal, so thatthere are no cracks in the sheathed-element glow plug.

What is claimed is:
 1. Sheathed-element glow plug to be mounted in acombustion chamber, having a housing (3) and a rod-shaped heatingelement situated in a concentric bore hole of the housing, the heatingelement having a first current-carrying layer (11) and a secondcurrent-carrying layer (12), the first current-carrying layer (11) andthe second current-carrying layer (12) being connected at the end of theheating element on the combustion chamber side via a conducting-layercrosspiece (13), the first current-carrying layer (11) and the secondcurrent-carrying layer (12) being separated by a first insulating layer(15), the first current-carrying layer (11) and the secondcurrent-carrying layer (12) being different lengths, wherein the crosssection of the first current-carrying layer (11) in a first section (21)at the end of the heating element away from the combustion chamber isgreater than the cross section of its remaining length, and the secondcurrent-carrying layer (12) does not extend into the first section (21).2. The sheathed-element glow plug as recited in claim 1, wherein thecross section of the first current-carrying layer (11) in the firstsection (21) corresponds to the cross section of the heating element. 3.The sheathed-clement glow plug as recited in claim 2, wherein the crosssection of the first insulating layer (15) in a second section (22) isgreater than the cross section of its remaining length, the secondsection (22) bordering first section (21) in the direction of thecombustion chamber, and the second current-carrying layer (12) notextending into the second section (22).
 4. The sheathed-element glowplug as recited in claim 3, wherein the end of the second section (22)of the heating clement on the combustion chamber side is not situated atthe junction (19) between a heating-element collar and a heating-elementshaft.
 5. The sheathed-element glow plug as recited in claim 1, whereinthe first current-carrying layer (11) is surrounded by an externalsecond insulating layer (18) over the length of a third section (23) ofthe heating element.
 6. The sheathed-element glow plug as recited inclaim 5, wherein the third section (23) of the heating element extendsfrom the end of the heating element away from the combustion chamberbeyond the end of the housing (3) away from the combustion chamber. 7.The sheathed-element glow plug as recited in claim 1, wherein the firstcurrent-carrying layer (11) has a stepped lug (11′) at the end of thefirst section (21) of the heating element away from the combustionchamber.
 8. The sheathed-element glow plug as recited in claim 1,wherein the first current-carrying layer (11), the secondcurrent-carrying layer (12), and the conducting-layer crosspiece (13)are made of electrically conducting ceramic material, and the firstinsulating layer (15) and the second insulating layer (18) are made ofan electrically insulating ceramic material.
 9. The sheathed-elementglow plug as recited in claim 8, wherein the first insulating layer (15)and the second insulating layer (18) are made of the same electricallyinsulating ceramic material.